Controlled Field Trial of Pancreas Disease Vaccines in Farmed Atlantic Salmon: Effects on Growth and Mortality During a <scp>SAV2</scp> Outbreak

Pancreas disease (PD), caused by salmonid alphavirus (SAV), is a concern for salmon aquaculture in Scotland, Ireland and Norway. This article presents results from a controlled field trial monitoring three study groups from vaccination to harvest. The groups represented a subset of the total fish population within a single sea cage, reared under commercial farming conditions. The trial aimed to evaluate the effects of two licensed PD vaccines in Norway-ALPHA JECT micro 1 PD (AJm1PD) and Clynav-on growth and mortality during a PD outbreak, relative to an unvaccinated control group. Clinical PD, caused by SAV subtype 2, was confirmed in June 2024, approximately 14 months after sea transfer and 2 months before harvest. At harvest, both vaccinated groups exhibited significantly higher weights compared to the control group, with mean increases of 0.19 kg (95% CI: 0.05-0.33) for AJm1PD and 0.38 kg (95% CI: 0.24-0.51) for Clynav. Significant risk differences (%) in mortality were observed between the PD-vaccinated groups and the control during the outbreak, with lower mortality in vaccinated fish (AJm1PD: -0.60 [95% CI: -0.98 to -0.22]; Clynav: -1.10 [95% CI: -1.45 to -0.74]). To account for potential misclassification of mortalities, bias-adjusted estimates were calculated, revealing greater risk differences compared to unadjusted estimates (AJm1PD: -0.99 [95% CI: -1.53 to -0.45]; Clynav: -2.07 [95% CI: -2.58 to -1.56]). These findings suggest that failing to adjust for misclassification biases the results toward the null, leading to an underestimation of the true effect size. In contrast, risk ratios remained largely unaffected by these adjustments. Overall, the study suggests that both vaccines improved outcomes, with the DNA vaccine (Clynav) showing a more pronounced effect on both growth and survival compared to the inactivated SAV vaccine (AJm1PD), while also highlighting methodological challenges in conducting controlled field trials.